PSI - Issue 48

Youcef Cheikhaoui et al. / Procedia Structural Integrity 48 (2023) 81–87 Cheikhaoui et al/ Structural Integrity Procedia 00 (2023) 000 – 000

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. After the execution of a mining operation in the massif, a redistribution of the stresses occurs around this mining structure. During this redistribution process, rocks tend to a new equilibrium state, undergoing certain deformations. Therefore, the study of stability in fractured rock massifs, as well as the modeling of mining works require the knowledge of behavior models and mechanical properties of these structures(Fan and Liu, 2017; Scholtès and Donzé, 2012). These mechanical properties are influenced by scale effects and even depend on the volume considered (Brady and Brown, 2004; Morlier et al., 1989). This effect is defined by the influence of the sample size on the measurement of a supposedly intrinsic size (Cheikhaoui et al., 2021a). In light of the evidence of constant strength beyond a critical size, particularly the "geometry" effect, the empirically observed decrease in volume strength would be a function of factors in the pillar system, rather than a material property of the pillar. In this context, we aim to attest how is the evolution of the characteristic strength of a pillar in an underground mine. The characteristic strength is understood in terms of the probability of exceeding a certain value of compressive strength. An analytical formulation was used which was developed by Cheikhaoui et al (Cheikhaoui et al., 2021a, 2021b) combining the Wei bull’s approach(Weibull, 1939) which takes into account scale effects and Galvin's approach(GALVlN et al., n.d.) which takes into account the shape effects. The objective of this paper is to study the influence of the effect of the shape and the stress applied to evaluate the characteristic values of the mechanical properties of the excavated rock mass. Moreover, even to specify the probability of failure in a perspective of dimensioning of the structures, or management of the risks associated with the extraction of the resources (mines, quarries, boreholes, etc.), or with the development of the underground (rock cuttings, tunnels, etc.). The evaluation of the characteristic resistance of a pillar will constitute the structuring case study of the paper. As a case study, we have taken the case of the Chaabet El Hamra mine in the city of Setif, Algeria. 2. Behavior of a pillar under uniaxial loading 2.1. Failure modes of pillar There is a great variation in the behavior of pillars at failure from one mine to another and even within a given mine. This behavior is related to the shape of the pillars, the nature of the rock and the local geo-mechanical conditions. According to (Brady and Brown, 2004), there are five main types of failure, figure 1, describe by (Brown, 1970; Diederichs, 2002; Lunder and Pakalnis, 1997).  Cracking and detachment of lateral flakes out: in the case of a relatively massive pillar/wall rock assembly with no marked discontinuity, Figure 1.a  An inclined through shear strip: in the case of pillars with a large launch figure 1.b  Pillar dilatation produced by sliding along interfaces: in the case where pillars and surrounding rock are separated by an interface marked by low shear strength, Figure 1.c  The failure is oriented along the planes of weakness constituted by the discontinuities: in the case of pillars cut by one or more families of fractures, figure 1.d  An axial separation mechanism: in the case of pillars marked by the presence of schistosity or foliation planes parallel to their height figure1.e.

Fig. 1. Failure modes of mine pillars according to their geomechanical conditions (Brady and Brown, 2004).